Various approaches have been described for performing assays to determine the presence or concentration of a specific analyte, such as a food or soil contaminant. Many assays determine the presence or concentration of analyte in a sample by relying on the binding of an analyte by another molecule or group of molecules. Examples of such molecules include but are not limited to analyte receptors, antibodies to the analyte, and other molecules and combinations of molecules with binding affinity for the analyte. One type of assays is the immunoassay, which is based on the highly specific binding between an antibody and an antigen recognized by the antibody.
When a binding assay is utilized to determine the presence or concentration of an analyte in a sample, it is often desirable to conduct further analysis to confirm the accuracy of the “positive” indication and rule out problems such as crossreactivity, which could lead to an inaccurate positive result. For example, immunoassays are often used in lateral flow devices such as test strips to determine the presence in foods of contaminants such as pathogens. However, additional confirmation of a positive test result is sometimes desired and in some cases required by governmental regulation to prevent the unnecessary recall and destruction of large quantities of the substance being tested, such as food products.
In some cases, the first step for confirmation of a positive result is to obtain a large quantity of the substance that is causing the positive result, possibly in pure state and free from similar microorganisms. For example, in some cases in which the contaminant of a food sample is a biological pathogen or microorganism (for example, a protozoan, bacteria, or virus) the contaminant can be cultured using conventional microbiological techniques to produce large quantities of the pathogen for subsequent analysis. However, suspected pathogens are often present among other non-pathogenic microbes in the sample, thus complicating the analysis. Obtaining a pure isolate or a sufficiently high concentration of an analyte such as a pathogenic contaminant can be tedious, expensive and time consuming.
Microorganisms in contaminated food that cause gastrointestinal infections are extremely common worldwide. Food contamination, often referred to as food poisoning, can have symptoms such as nausea, vomiting, abdominal pain, diarrhea and fever. If left untreated, some contaminants can cause death, especially in children, the elderly, or in immunocompromised individuals. Food contaminants such as some varieties of Staphylococcus aureus and Bacillus cereus can produce enterotoxins, which can cause symptoms such as nausea and vomiting. Some viruses such as rotaviruses and Norwalk virus, and some protozoans such as Giardia lamblia can cause diarrhea by irritating or destroying the intestinal mucosa. Dysentery, an infection of the colon that can cause abdominal pain, hemorrhagic diarrhea, fever and dehydration can be caused by organisms such as Salmonella, Shigella, Campylobacter, Escherechia coli, and Clostridium. E. coli O157 is a particularly pathogenic strain of E. coli that has been found to be a contaminant of meat and other food and beverage products.
One method for confirming a positive test result for the presence of Escherechia coli O157 in a sample of food or beverage involves culturing microorganisms in the sample to produce large quantities. The microorganisms are then separated from each other and from other components of the culture. One technique for performing this procedure is the use of magnetic beads coated with an antibody specific for E. coli O157. The culture is poured over a column containing the beads and absorbed to the surface of the beads. Beads are then collected using a magnet, washed, and eluted to obtain a solution in which any E. coli present will have a concentration higher than the original sample. The high concentration solution can then be streaked on selective agar plates for growth and inspection to determine the presence of E. coli O157 colonies. This procedure is not only expensive, but is also time-consuming, an important consideration when confronted with a decision to suspend or to recall distribution of a widely distributed food product.
Thus, what is needed in the art is a rapid method for confirming the results of an assay. What is further needed in the art is a rapid method for obtaining a sample in which the concentration of the microbes, molecules, or other objects that caused a “positive” indication of an assay is increased as compared to an original sample.